Multiple c-myb transcript cap sites are variously utilized in cells of mouse haemopoietic origin

Mouse c-myb gene transcripts in various cells of haemopoietic origin were analysed using S1 nuclease and RNase mapping techniques and by Northern blotting. It was found that the prevalent 3.8-kb c-myb mRNA present in thymocytes, T cell leukaemias, myelomonocytic leukaemias, erythroleukaemias and myeloid stem cells was initiated at several cap sites mapping within a region 97-244 bp upstream from the protein coding sequence. Utilization of additional cap sites mapping further upstream was also observed in certain cells, most notably thymocytes, and this gave rise to RNA species (4.3-5.6 kb) larger than the presumptive mRNA. In contrast, myeloma cell c-myb transcripts, which are much less abundant than those in more immature haemopoietic cells, were found to be initiated at a restricted set of cap sites mapping 244-277 bp upstream of the coding sequence. Hence, these data suggest that the abundance of the c-myb mRNA may be regulated by a process involving selective utilization of mRNA cap sites. Sites hypersensitive to DNase I were associated with mRNA cap sites in cells that expressed c-myb.